I have heard that there are some apps that hook into the Apple processes that will run in the background (namely, hooking into the Apple clock timer API), which will allow some non-UI background process on your 3rd party app to continue to run. Is this legitimate? Supposedly, there are some GPS-tracker apps that are using this and have passed Apple review and are for sale in the app store.Can I realize this and how can I do?Is there any API for this?

Even if there is a way, you will not get through the AppStore review process. They now use an automated search for private API calls and automatically reject any apps that they find to be using private APIs. Further, you are not allowed to run your app in the background, if they discover that you have managed to do this, your app will be rejected.